This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
`IHasComboColours` was already mutable (via a strange
`AddComboColours()` method) and exposing a straight list is easier to
work with. `IHasCustomColours` is also similarly externally mutable (in
a way which is not easily removable).